Theory of chemical bonds in metalloenzymes II: Hybrid-DFT studies in iron–sulfur clusters



Important chemical reactions for life often require multistep electron transfers (ET) and strong reducing forces. In these reactions, electron transfer proteins as ferredoxins (Fds) play a key role. For elucidation of the core electronic states in these electron transfer processes, an inorganic model compound [Fe2S2(S2-o-xyl)2] is used as our first study. It was experimentally characterized that the model compound is in a similar electronic state to the active site core in Fds. On the reduced form, the diiron core exists in a characteristic mixed-valence state that has mobile electron (spin). Hybrid density functional theory (HDFT) calculations are performed to investigate the chemical bond nature, electronic structures, and magnetic interactions. The spin states and energy levels are further discussed with spin Hamiltonians, which contain Heisenberg exchange term and double exchange term to describe the mixed-valence state. We have determined their effective exchange integrals (J) and resonance parameters (B) from (HDFT) calculations in several procedures. These magnetic interactions are in good agreement with experiments. To estimate B values, we propose a new procedure using molecular orbital energies. The B values are properly evaluated compared with other procedures, using total energies. The chemical bond natures and the ground electronic structures are elucidated in terms of chemical indices defined by the occupation number of natural orbitals. Finally, implications of the computational results are discussed in relation to rational design of biomolecular devices. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005